1. Field of the Invention: This invention relates in general to methods and apparatus for joining pipe ends and in particular to methods and apparatus for joining large diameter pipe ends without the necessity of rotating the pipe.
2. Description of the Prior Art: In the early 1960's oil exploration and production went offshore, and the limitations of the technical possibilities of standard A.P.I. threaded or flanged connections became apparent because of the increase in diameter of the connection required in offshore operation, the requirement of being able to make-up and disconnect underwater connections remotely, and the motions of the floating support where most of the offshore operations were taking place. Also the cost of field welding widely used onshore to connect sections of large pipe, became prohibitive due to the high hourly cost of offshore operations. In those early days, three types of connectors were introduced to solve these problems.
First, a specialized generation of thread connectors were designed, using modified A.P.I. conical thread, to extend the use of threaded casing connections larger than sixteen inches for operation from a floating support. These threaded connectors are still widely used today, for dimensions below twenty-four inches, despite the requirement of a minimum of two and one-half turns of rotation for make-up.
This rotation is commonly achieved either by use of a hydraulic power tong which requires a specialized crew to operate, or the conventional "rope technique" consisting of pulling on a soft rope from the drilling rig main winch capstan. The rope is rigged three to five turns around the add-on casing piece body, and the pull rotates the pin and box members toward each other. Final make-up torque is reached by the use of a mechanical tong consisting of a large chain wrench having a lever extending radially to multiply the torque generator by the cathead chain hooked onto the end of the lever. The other end of the cathead chain is pulled by a drilling rig main winch.
The maximum amount of rotation achieved in one stroke of such a mechanical tong/cathead chain assembly is approximately one-quarter to one-third of a turn without the necessity of having to reset the assembly by manual reverse rotation.
A threadless second connector for large diameter casings was also introduced based upon snap-ring linkage. These "snap-ring" type connectors offer fast make-up but offer neither the low weight-to-capacity ratio, nor the rigidity or price competitiveness of the threaded connector for a given capacity.
A third connector, featuring radial dogs, was introduced to replace flange connections, in a mechanical configuration for drilling riser connections, and in a hydraulic configuration for wellhead connections.
Recently, the use of dog type connectors has been extended to offshore platform anchoring pile connections, but its success has been limited by its high cost, due to the large number of parts and heavy section required in the design and manufacture of such a connector.
In the 1970's, two additional types of connectors were introduced. One type of connector featured large dimensioned threads or circular grooves which are cylindrical or conical. Such threads or grooves are milled out on at least two sections limited by cylindrical or conical generatrix, in order that make-up of the connector can be achieved by axial stabbing, followed by a rotation of a fraction of a turn to bring the remaining portions of threads or grooves into an interlocking configuration. The make-up rotation is limited to a maximum of one-quarter of a turn.
The major drawback experienced with this type of connector, despite its fast make-up characteristics, is the cost of manufacturing, due to the extensive milling operation and the capacity-to-weight ratio, due to the removal of fifty percent of the loading surfaces.
A second type of connector introduced during this period is a no rotation "interference type" connector mainly designed for permanent application such as an offshore platform conductor pipe or anchoring pile. It presents the major drawbacks of not being disconnectable and requiring a special bulky hydraulic clamping unit, operated by a specialized crew, for make-up.
Recently a method and apparatus for making tubular connections was disclosed in U.S. Pat. No. 4,717,183, and assigned to the assignee herein, which utilizes multiple start threads to provide a relatively strong and rapid make-up between two pipe members. While this technique does represent an advance in the art, it still requires rotation of the pipe members to make the joint. The large diameter of pipes being utilized in the modern offshore drilling and production make it desirable to implement a system which permits the making of a joint without the necessity of pipe rotation, such as the connections for large diameter tendons on tension leg platforms.